1. Field of the Invention
The present invention relates to a membrane unit, an apparatus comprising a membrane unit and a method for decontaminating blood, by removing metabolite contaminates, i.e., toxic metabolites and metabolites normally present in urine.
2. Description of the Prior Art
Dialysis, diafiltration and hemoperfusion are known methods for removing toxic metabolites from blood. By metabolites are meant those components of the living cells which control the normal course of metabolic reactions, as well as products of metabolism formed or catabolized in human or animal organisms, such as urea, creatinine, peptides, carbohydrates and electrolytes, e.g., sodium or potassium salts. In dialysis and diafiltration separation is achieved by means of filtration via selectively permeable membranes hereinafter referred to as permselective membranes. Hemoperfusion is based on the principle of adsorption.
In the dialytic process the substances to be eliminated, namely water and substances normally present in urine, that is, uric acid, urea, creatinine, carbohydrates, electrolytes and peptides, are removed through a permselective membrane which exchanges the contaminants with a rinsing solution containing some of the substances which are vital to the organism. The exchange of diffusible substances across the permeable membrane is caused by the difference in concentrations on each side of the membrane.
The driving force in the diafiltration process is an adjustable pressure gradient. Any substance of a size below the porosity limit of the permselective membrane is pressed out as an ultrafiltrate in the same ratio of concentration as in blood. The ultrafiltrate may be rejected; however, a certain portion of the ultrafiltrate extracted from the blood must be returned to the bloodstream with all vital substances in a physiological ratio of concentration. Dialysis and diafiltration find primarily utility in treating persons suffering from chronic kidney diseases.
Conversely, hemoperfusion is based on a different principle of operation, and its application has up to now been almost exclusively limited to cases in which a paticularly rapid detoxification of the blood is required; as in acute failures of the liver or intoxications. In this method, adsorbents, such as activated carbon or macroporous resins are used to adsorb toxic metabolites. The adsorbents, which are usually enveloped by a porous membrane material, are generally used in granulated form, enclosed as micro-capsules in an aqueous suspension, coated upon support webs or used as fiber bundles disposed in columns through which contaminated blood passes. The enveloping of the adsorbents in a porous membrane material is preferred since it prevents direct contact with the blood which improves blood compatibility. Nevertheless, there is still a considerable risk of damaging the blood, particularly from loss of blood cells and proteins, from micro-embolisms due to washed-out adsorbent particles, and from an interruption of the steady flow in the column passed by the blood. In view of this high risk, the use of hemoperfusion is limited to cases in which the patient is comatose.
An additional disadvantage of the hemoperfusion systems is that the adsorbent is not adequately capable of adsorbing all metabolites normally present in urine, such as water, urea, electrolytes and ammonia. Even the use of additional complex and expensive measures, such as the inclusion of enzymes in micro-capsules, do not result in satisfactory removal of these metabolites.
It has heretofore been suggested to combine diafiltration and hemoperfusion by connecting corresponding devices in series in order to utilize the rapid detoxifying action of hemoperfusion and to extract the non-adsorbable metabolites normally present in urine. This procedure is, however, disadvantageous due to the high blood-filling and residual volume of the devices; in particular, the patient is exposed to the risk of hypotension. Moreover, in addition to the problems inherent in the use of the hemoperfusion device previously described, there is the danger of damaging the blood due to the use of additional, complex apparatus. Finally, it is difficult to coordinate the devices, and further their operation is extremely expensive.
It has also been proposed to pass the ultrafiltrate resulting from diafiltration over toxin-adsorbing substances and subsequently return the detoxified filtrate to the blood stream. The devices operating according to this procedure have, in particular, the disadvantage that only the ultrafiltrate which is separated from the blood, and not the bulk of the blood is contacted with the adsorbent. Thus, there remains a need in the art for a simple but efficient means of removing metabolite contaminants from blood that avoids the dangers and problems associated with the existing means.